U.S. Pat. No. 2,053,822 discloses a process for the manufacture of sulphonic acids, consisting in treating with sulphonating agents imidazole derivatives having the atom grouping in which R represents a carbocyclic aromatic radical containing at the most 10 carbon atoms, R1 represents an alkyl radical containing at least 7 carbon atoms and R2 is a member selected from the group consisting of hydrogen and alkyl. None of the alkylating agents cited contains an aromatic group i.e. benzyl chloride is apparently not included. The aromatic nucleus R may comprise substituents, for example, sulpho groups. The 2-substituent may also be a methoxy-, ethoxy-, mercapto- or thioalkyl- group. U.S. Pat. No. 2,053,822 further discloses that new imidazoles are characterized by the atom grouping wherein the radical R represents a sulphonated aromatic radical, wherein further the nature of the μ-substituent follows from the above enumerations, and wherein at least one of the two nitrogen atoms is linked with a radical R1. The examples disclose the reaction of dodecyl chloride, 2-chloroacetic acid dodecyl ester with μ-heptadecyl-benzimidazole followed by sulphonation.
Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
U.S. Pat. No. 3,704,130 discloses a method of preparing a photographic fine-grain silver halide emulsion, which comprises the step of precipitating the silver halide in an aqueous hydrophilic colloid medium in the presence of a compound corresponding to the following general formula: Z—A—X, wherein: each of Z and X (the same or different) stands for a heterocycle or a heterocycle with fused-on ring, said heterocycle comprising the moiety ═N—, and A stands for a chemical bond, alkylene, alkylene interrupted by oxygen or —N(R)— wherein R=hydrogen or alkyl comprising at most 4 C-atoms, arylene, alkenylene, —S-alkylene-S— or —S-alkylene, the alkylene groups of which can be interrupted by oxygen or —N(R)— wherein R has the same significance as above; said compound being present in an amount sufficient to restrain growth of silver halide grains. U.S. Pat. No. 3,704,130 discloses the following 2-alkylthio-benzimidazole-6-sulphonic acid compounds as Compound 7: and Compound 9: 
U.S. Pat. No. 4,639,408 discloses a process for forming an image comprising a heating step wherein a silver halide light-sensitive photographic material is heated in the presence of a compound represented by formula (I) wherein X represents an atomic group completing a carbocyclic aromatic ring or a heterocyclic aromatic ring; R1 is selected from selected from groups represented by formulae (A), (B), and (C):—SO2—R11  (A)—C(═O)—R11  (B)—P(═O)R11R12  (C)in which R11 and R12 each represents a substituted or unsubstituted alkyl group, a cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, an alkoxy or aryloxy group, an alkylthio or arylthio group, or a substituted or unsubstituted amino group, or R11 and R12 together form a 5-membered or 6-membered ring; R2 represents a group selected from the groups represented by R11; R1 and R2 together form a 5-membered or 6-membered ring; Q represents a hydrogen atom, an alkyl group, or an aryl group; TIME represents a timing group; PUG represents a photographically useful group; and n represents 0 or an integer, wherein the photographic material contains a base or a base precursor. U.S. Pat. No. 4,639,408 discloses the following 2-alkylthio-benzimidazole-6-sulphonic acid compounds as Compound (8): Compound (14): Compound (24) Compound 25: and Compound 29: 
Surfactants with excellent latex-stabilizing properties, which enhance the adhesion of hydrophilic layers to hydrophobic supports, such as polyethylene terephthalate and hydrophobic thermosensitive elements of substantially light-insensitive thermographic recording materials, which are photographically inactive, which do not contain photographically inactive impurities and which is compatible with image-wise heating with a thermal head when incorporated into the outermost layer of substantially light-insensitive thermographic recording materials.
ULTRAVON™ W, an anionic alkyl-, benzyl-benzimidazole-sulfonic acid surfactant produced by Ciba, fulfils these requirements, except that being a mixture of at least 25 components it has variable properties and also often contains photographically active impurities, which have to be removed or rendered non-photographically active prior to use. Furthermore, it is no longer available.